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New issue from Jonathan: make_shared should not refer to a type U[N] for runtime N
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‎xml/issue4451.xml‎

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<?xml version='1.0' encoding='utf-8' standalone='no'?>
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<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
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<issue num="4451" status="New">
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<title>`make_shared` should not refer to a type `U[N]` for runtime N</title>
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<section><sref ref="[util.smartptr.shared.create]"/></section>
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<submitter>Jonathan Wakely</submitter>
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<date>05 Nov 2025</date>
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<priority>99</priority>
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<discussion>
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<b>Addresses <a href="https://github.com/cplusplus/nbballot/issues/718">US 76-139</a></b>
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<p>
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The overloads of `make_shared` and `allocate_shared` for creating
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<code>shared_ptr&lt;T[]&gt;</code> refer to an object a type `U[N]`
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where `N` is a function parameter not a constant expression.
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</p>
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</discussion>
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<resolution>
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<p>
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This wording is relative to <paper num="N5014"/>.
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</p>
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<ol>
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<li><p>Modify <sref ref="[util.smartptr.shared.create]"/>, as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class T&gt;
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constexpr shared_ptr&lt;T&gt; make_shared(size_t N); <i>// T is U[]</i>
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template&lt;class T, class A&gt;
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constexpr shared_ptr&lt;T&gt; allocate_shared(const A&amp; a, size_t N); <i>// T is U[]</i>
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</pre>
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<blockquote>
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<p>-12- <i>Constraints</i>:
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`T` is
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<del>of the form `U[]`</del>
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<ins>an array of unknown bound</ins>.
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</p>
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<p>-13- <i>Returns</i>:
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A `shared_ptr` to an <del>object of type `U[N]`</del>
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<ins> array of `N` elements of type <code>remove_extent_t&lt;T&gt;</code></ins>
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with a default initial value<del>,
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where `U` is <code>remove_extent_t&lt;T&gt;</code></del>.
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</p>
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<p>-14- [<i>Example 2</i>: ...]</p>
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</blockquote>
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<pre>
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template&lt;class T&gt;
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constexpr shared_ptr&lt;T&gt; make_shared(); <i>// T is U[N]</i>
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template&lt;class T, class A&gt;
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constexpr shared_ptr&lt;T&gt; allocate_shared(const A&amp; a); <i>// T is U[N]</i>
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</pre>
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<blockquote>
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<p>-15- <i>Constraints</i>:
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`T` is <del>of the form `U[N]`</del>
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<ins>an array of known bound</ins>.
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</p>
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<p>-16- <i>Returns</i>:
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A `shared_ptr` to an object of type `T` with a default initial value.
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</p>
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<p>-17- [<i>Example 3</i>: ...]</p>
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</blockquote>
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<pre>
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template&lt;class T&gt;
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constexpr shared_ptr&lt;T&gt; make_shared(size_t N,
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const remove_extent_t&lt;T&gt;&amp; u); <i>// T is U[]</i>
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template&lt;class T, class A&gt;
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constexpr shared_ptr&lt;T&gt; allocate_shared(const A&amp; a, size_t N,
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const remove_extent_t&lt;T&gt;&amp; u); <i>// T is U[]</i>
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</pre>
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<blockquote>
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<p>-18- <i>Constraints</i>:
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`T` is
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<del>of the form `U[]`</del>
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<ins>an array of unknown bound</ins>.
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</p>
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<p>-19- <i>Returns</i>:
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A `shared_ptr` to an <del>object of type `U[N]`</del>
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<ins> array of `N` elements of type <code>remove_extent_t&lt;T&gt;</code></ins>
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where
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<del>`U` is <code>remove_extent_t&lt;T&gt;</code> and</del>
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each array element has an initial value of `u`.
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</p>
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<p>-20- [<i>Example 4</i>: ...]</p>
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</blockquote>
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<pre>
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template&lt;class T&gt;
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constexpr shared_ptr&lt;T&gt; make_shared(const remove_extent_t&lt;T&gt;&amp; u); <i>// T is U[N]</i>
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template&lt;class T, class A&gt;
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constexpr shared_ptr&lt;T&gt; allocate_shared(const A&amp; a,
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const remove_extent_t&lt;T&gt;&amp; u); <i>// T is U[N]</i>
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</pre>
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<blockquote>
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<p>-21- <i>Constraints</i>:
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`T` is <del>of the form `U[N]`</del>
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<ins>an array of known bound</ins>.
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</p>
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<p>-22- <i>Returns</i>:
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A `shared_ptr` to an object of type `T`, where each array element of type
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<code>remove_extent_t&lt;T&gt;</code> has an initial value of `u`.
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</p>
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<p>-23- [<i>Example 5</i>: ...]</p>
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</blockquote>
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<pre>
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template&lt;class T&gt;
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constexpr shared_ptr&lt;T&gt; make_shared_for_overwrite(); <i>// T is U[N]</i>
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template&lt;class T, class A&gt;
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constexpr shared_ptr&lt;T&gt; allocate_shared_for_overwrite(const A&amp; a); <i>// T is U[N]</i>
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</pre>
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<blockquote>
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<p>-24- <i>Constraints</i>:
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`T` is not an array of unknown bound.
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</p>
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<p>-25- <i>Returns</i>:
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A `shared_ptr` to an object of type `T`.
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</p>
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<p>-26- [<i>Example 6</i>: ...]</p>
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</blockquote>
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<pre>
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template&lt;class T&gt;
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constexpr shared_ptr&lt;T&gt; make_shared_for_overwrite(size_t N); <i>// T is U[]</i>
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template&lt;class T, class A&gt;
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constexpr shared_ptr&lt;T&gt; allocate_shared_for_overwrite(const A&amp; a, size_t N); <i>// T is U[]</i>
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</pre>
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<blockquote>
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<p>-27- <i>Constraints</i>:
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`T` is an array of unknown bound.
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</p>
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<p>-28- <i>Returns</i>:
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A `shared_ptr` to an <del>object of type `U[N]`</del>
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<ins> array of `N` elements of type <code>remove_extent_t&lt;T&gt;</code></ins>
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<del>, where `U` is <code>remove_extent_t&lt;T&gt;</code></del>.
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</p>
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<p>-29- [<i>Example 7</i>: ...]</p>
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</blockquote>
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</blockquote>
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</li>
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</ol>
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</resolution>
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</issue>

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